Mobile assisted hard hand-off for a code division multiple access (CDMA) system

ABSTRACT

A method and system for mobile assisted hand-off between base stations using different carrier frequencies in a Code Division Multiple Access (CDMA) cellular system. In the method and apparatus, a mobile station performs handoff measurements on forward link transmissions of handoff candidate CDMA base stations, where the forward link transmissions are transmitted on a carrier frequency that differs from the forward link carrier frequency of the current base station. The handoff measurements are performed during selected transmission frames that define transmission periods on the reverse link by the mobile station. During the measurement time in the selected transmission frame, the mobile station does not receive (punctures) data sent to it on the forward link, and does not transmit data on the reverse link.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to handoff techniques in cellulartelecommunication systems, and more particularly to a method andapparatus for Mobile Assisted Hand-off between different frequencies ina Code Division Multiple Access (CDMA) cellular system.

2. Background Art

Each base station in a cellular system provides a radio interface tomobile stations within a particular area or zone of the system generallyreferred to as a cell. Each base station is in communication with amobile switching center which may switch calls from one base station toanother base station and connects the base station with the local phonesystem over land lines or connects the base station with anothercellular telephone system. One of the most critical functions associatedwith mobile telephone use in a cellular system is the transfer of callsfrom one base station to another as the user travels from one cell to anadjoining cell. This transfer of call control is commonly known ashand-off.

As the mobile telephone moves through a cell, the strength of itstransmitted signal will vary depending on its distance from the basestation. The base station's range within its cell is designed to dropoff rapidly as the coverage range of its cell is reached. If a mobiletelephone leaves a cell during ongoing communication and moves into adifferent cell, it is necessary that a new base station be selectedwithout any disturbance to the communication. In early analog systems,the current base station monitored the strength of the signal emanatingfrom the mobile telephone. As the signal strength lowered to criticallevels, a search was conducted of neighboring base stations to determinewhich base station was receiving the strongest signal from the movingmobile telephone. The switching center would then instruct the mobiletelephone through the current base station to change to the necessaryparameters of the new base station and handoff occurred.

In more recent Time Division Multiple Access (TDMA) systems where usersare each assigned unique time slots on a frequency channel, the mobilestations are designed to measure the strength of certain signalstransmitted by surrounding base stations during timeslots whencommunications are inactive and transmit this information back throughthe base station to the switching center. This provides the switchingcenter with the information necessary to decide when and to what basestation a particular mobile telephone must be switched in order tomaintain communication. This technique is called Mobile Assisted Handoff(MAHO).

The most recent type of cellular technology to be developed is CodeDivision Multiple Access (CDMA). In a CDMA type system multiple users,each using a channel identified by a uniquely assigned digital code,simultaneously communicate with the system while sharing the samewideband frequency spectrum. A CDMA system may be designed so that allbase stations share the entire downlink frequency spectrum and allmobile stations share the entire uplink frequency spectrum. In a CDMAsystem, then, handoff may be necessary between base stations using thesame frequencies. This is known as "soft handoff". In soft handoff amobile station in transition from one base station to another transmitsand receives the same signal from both base stations simultaneously. Ina CDMA receiver a rake receiver can be used to isolate the signalsreceived at the mobile station from each base station and align themboth in time and phase to reinforce one another on the base to mobilelink. On the reverse link a mobile switching center connected to bothbase stations resolves which base station is receiving the strongersignal and selects the signal from that base station for the call.

In a CDMA system soft handoff may not always be possible. In these caseshard handoff is used. Hard handoff is handoff in which a temporarydisconnection of the communication path (i.e. traffic channel) between amobile station and a base station takes place. Hard handoffs occur whenthe mobile station is transferred between disjoint active sets, the CDMAfrequency assignment changes, the frame offset changes, or the mobilestation is directed from a CDMA traffic channel to an analog voicechannel. Hard handoff is distinct from soft handoff whereincommunications with a new base station is commenced on the same CDMAfrequency assignment before the communications with the old base stationare terminated. Hard handoff presents some difficulties in CDMA systems.In general, in a CDMA system a user is assigned all data transmissionframes at a CDMA traffic channel sequentially in time. In mobileassisted hard handoff, since the frequencies differ between the systemsinvolved, it would be necessary to use a separate receiver to makehandoff measurements at the mobile station, if the mobile assistedhandoff were to be non-interfering with ongoing communications.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a Code DivisionMultiple Access (CDMA) cellular system that permits Mobile AssistedHand-Off (MAHO) between CDMA base stations that use different carrierfrequencies by performing periodic searches of the different carrierfrequency base station pilots.

Another object of the present invention is to provide a mobile CDMAcellular system wherein the mobile telephone searches for the basestations on a different carrier frequency without significantdegradation in system performance.

The present invention provides a method and system for mobile assistedhard hand-off (MAHHO) between base stations using different carrierfrequencies in a Code Division Multiple Access (CDMA) cellular system.In the method and apparatus, a mobile station performs periodic handoffmeasurements on forward link transmissions of handoff candidate CDMAbase stations, where the forward link transmissions are transmitted on acarrier frequency that differs from the forward link carrier frequencyof the current base station. The handoff measurements are performedduring selected transmission frames that define transmission periods onthe reverse link by the mobile station. During the measurement time inthe selected transmission frame, the mobile station does not receive(punctures) data sent to it on the forward link, and does not transmitdata on the reverse link.

In an embodiment of the invention, transmission frames including datathat is being transmit at a selected reduced rate are used as theselected transmission frames for mobile station handoff measurements.The position of the measurement periods within the selected transmissionframes may be chosen to minimize the amount of data that is nottransmitted and lost on the reverse link during the measurement period.The position of the measurement period may also be chosen so thatdisturbance of other functions of the system, such as, for example,mobile station power control from a base station, is minimized. Duringthe measurement periods the mobile station punctures the received datastream that is transmitted to the mobile station on the forward link.

The handoff measurements may occur periodically. In one alternative thatmay be used for variable data rate transmissions, such as a voice callusing a variable rate coder, a periodically expiring timer causes thenext transmission frame, that includes data at the selected reducedrate, subsequent to the timer's expiration, to be chosen as thetransmission frame for handoff measurements. In another alternative thatmay be used for data transmissions having a data role settableindependent of the application, the data transmission rate isperiodically forced to a selected reduced rate in both the base stationand mobile station and, the handoff measurements are periodicallyperformed during the transmission frames that include the data at theselected reduced rate.

In an alternative embodiment of the invention, the selected transmissionframes for mobile station handoff measurements may be chosenperiodically, but without regard for the data rate of the selectedtransmission frame. In this embodiment the handoff measurements areperformed during the selected transmission frames on the reverse linkwith no attempt to optimize the position of the measurement periodwithin the selected transmission frame. Data to be transmitted on thereverse link during the selected transmission frame is not transmittedand lost. In this embodiment the mobile station punctures the receiveddata stream that is transmitted to the mobile station on the forwardlink.

BRIEF DESCRIPTION OF THE DRAWINGS

Other and further features, advantages and benefits of the inventionwill become apparent in the following description taken in conjunctionwith the following drawings. It is to be understood that the foregoinggeneral description and the following detailed description are exemplaryand explanatory but are not to be restrictive of the invention. Theaccompanying drawings which are incorporated in and constitute a part ofthis invention and, together with the description, serve to explain theprinciples of the invention in general terms. Like numerals refer tolike parts throughout the disclosure.

FIG. 1 is a block diagram illustrating an inter-frequency handoffscenario in a CDMA cellular system;

FIG. 2 is a block diagram showing portions of a CDMA mobile stationaccording to an embodiment of the invention;

FIG. 3 is a flow diagram illustrating handoff measurement process stepsaccording to an embodiment of the invention;

FIG. 4 is a timing diagram illustrating reverse link transmission andhandoff measurement timing according to an embodiment of the invention;

FIGS. 5A and 5B illustrate configurations for a MAHHO Neighbor ListMessage (MNLM) and a MAHHO Pilot Strength Measurement (MPSM) Message.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, therein is a block diagram illustrating aninter-frequency handoff scenario in a CDMA Cellular Network. System Acomprises base stations BA1 and BA2 and System B comprises base stationsBB1 and BB2. Mobile stations MS1, MS2 and MS3 are capable of operationin either System A or System B. System A uses carrier frequency f₁ forbase to mobile (forward) link and frequency f₂ for mobile to base(reverse) link communications with mobile stations, such as MS1 and MS2operating within system A. System B uses carrier frequency f3 forforward link and carrier frequency f4 for reverse link communications,with mobile stations such as MS3, operating within system B. In FIG. 1,MS1 is located near the coverage boundary between System A and System B.MS1 is engaged in communications with base station BA1 on trafficchannels of forward link f1 and reverse link f2 of System A. At the sametime the signal strength of signals received on the forward link f3 ofSystem B has become strong enough to be received at MS1. In the methodand apparatus of the invention, the pilot channels of base stations ofSystem B, that are transmitted on the forward link f3, are measured byMS1 as MS1 is simultaneously engaged in communications with System A onforward link f1 and reverse link f2. In the embodiment of the inventioncellular Systems A and B and mobile stations MS1, MS2 and MS3, mayoperate according to the Telecommunications Industry Association(TIA/EIA) IS-95A or ANSI-008 Standard for CDMA systems. With theexception being that the necessary signaling modifications are made tothe air interface to allow the mobile assisted hard handoff of theinvention.

Referring now to FIG. 2, therein is a functional block diagram showingportions of a CDMA mobile station MS1 according to an embodiment of theinvention. MS1 comprises antenna 202, RF receiver (RX) 204, rakereceiver 220, Buffer 222, Searcher 224, Viterbi decoder 226, digitalsignal processing/microprocessor (DSP/MCU) 228, earpiece (EAR) 238,microphone (MIC) 240, transmit control (TXCTL) 218, encoder (ENCODE)216, modulator/spreader (MODUL/SPREAD) 214, gating control 212, RFtransmitter (TX) 210, synthesizer 206 and mobile assisted hard handoffcontrol (MAHHO CTL) 208. DSP/MCU 228 includes MAHHO processor 236,vo-coder 234, MAHHO pilot measurement message processor 230, and pilotmeasurement message processor 232.

Signals received at antenna 202 are converted to an IF frequency,filtered and digitized by RX 204 and then inputted to rake receiver 220.Synthesizer 206 has a fast and a slow mode. MAHHO CTL 208 controlsselection of fast and slow mode through signals SYNTH FAST and SYNTHSLOW. In an IS-95 embodiment where synthesizer 206 must be compatiblewith narrow-band AMPS, synthesizer 206 may operate with three centerloops, each having a different loop gain. In the CDMA mode synthesizer206 operates in either fast or slow (normal) mode. In order to implementthe embodiment of the invention where MAHHO measurement, are made withinselected transmission frames on the reverse link, synthesizer 206 willsettle to within 300 KHZ of the frequency in less than 1 ms when in thefast mode. RX 204 has a fast automatic gain control mode and a slow(normal) automatic gain control mode. MAHHO (CTL 208 controls theselection of fast or slow automatic gain control through signals, AGCfast and AGC slow. In the fast mode the automatic gain control has astep size of 2 dB and a threshold at 0.25 dB. This allows the automaticgain control to settle to the signal step change of ±/40 dB, within 1ms. The slow modes for synthesizer 206 and RX 204 automatic gain controlmay be normally used for IS-95 mobile stations. The digitized IF signalinput to rake receiver 220 may include signals from many ongoing callstogether with pilot carriers transmitted by the base station with whichthe mobile station is currently communicating, plus pilot carrierstransmitted by neighboring base stations. The signals that are receivedsimultaneously must all be transmited at the same forward link carrierfrequency. Rake receiver 220 performs correlation on the IF signal witha PN sequence of a desired received signal. Searcher 224 scans the timedomain around the nominal time of a received pilot signal of a basestation for other multi-path pilot signals from the same base stationand for other signals transmitted from different base stations. Searcher224 measures the strength of these other signals at times other than thenominal time. Searcher 224 generates signals to MAHHO pilot processor230 and pilot processor 232 indicating the strengths of the measuredsignals. The output of rake receiver is inputted to Viterbi decoder 226.The encoded data signals are aligned and combined and the resultant datasignal is then decoded using error correction and input to vo-coder 236.Vo-coder 236 then outputs information signals to earpiece 238.

For transmission of signals from mobile station MS1, a signal receivedat microphone 240 or data input 242, is inputted to vo-coder 234 indigital form, as for example fax data received at data input 242, or, asvoice that has been received at microphone 240 and digitized. Invo-coder 234 the signal is processed and outputted to encoder 216.Encoder 216 walsh encodes the signal. From encoder 216 the walsh encodedsignal is fed to module spread 214 where it is modulated and spread ontothe PN carrier sequence or the CDMA reverse link system channel to whichMS1 is assigned. The modulated signal is inputted to TX 210 fortransmission from antenna 202.

MAHHO processor 236 of DSP/MCU processor 228 provides control signals toMAHHO CTL 208 for DSP/MCU 228. MAHHO CTL 208 controls synthesizer 206,RX 204, buffer 222 and searcher 224 during MAHHO. TCCTL 218 provides acontrol signal to gating control 212 during MAHHO.

Referring now to FIG. 3, there is shown a flow diagram illustrating thehandoff measurement process steps according to the embodiment of theinvention. The process shown in FIG. 3 is described with reference tFIG. 1 and an example of MAHHO for mobile station MSI handing off fromBase Station BA1 to base station BB1. The process beings at step 302 inFIG. 3 as mobile station MS1 communicates with base station BA1 onforward link f₁ and reverse link f₂ traffic channels of System A. Atstep 304 mobile station MS1 receives a MAHHO Neighbor List Message(MNLM) with MAHHO interval timer information from base station BA1 onthe traffic channel. The MNLM message is transmitted by BA1, when aMobile Stattion is operating in the coverage area of base station BA1because the coverage area of base station BA1 borders the system B whichuses different frequencies. FIG. 5A illustrates a possible configurationfor the MNLM message. Message type (MSG₋₋ TYPE) field indicates that themessage is a MNLM message Acknowledgement Sequence (ACK₋₋ SEQ) field.The sequence number is set by the base station to the message sequence(MSG₋₋ SEQ) of the most recently received reverse traffic channelmessage. The MSG₋₋ SEQ of this MNLM message is set by the base stationto the message sequence number for this message Acknowledgement required(ACK₋₋ REQ) field indicates whether an acknowledgement is required forthis message. The ENCRYPTION field is set to the current messageencryption mode. The pilot increment (PILOT₋₋ INC) field indicates thepilot PN/sequence offset index increment. The mobile station searchesfor remaining, set pilots at pilot PN sequence offset index values thatare multiples of this value. CNMA frequency (CDMA₋₋ FREQ field indicatesthe frequency assignment for the MAHHO measurement. The use time (USE₋₋TIME), indicator field indicates whether an action time (ACTION₋₋ TIME)is specified for a data call. If the USE₋₋ TIME field indicates anACTION₋₋ TIME is specified, the ACTION₋₋ TIME field is set to the systemtime, in units of 80 ms (module 64), at which the frequency specified inthe CDMA₋₋ FREQ will be searched to set the MAHHO internal timer. Theaverage search period (AVER-SRCH₋₋ PRIOD) field specifies the averageperiod at which the mobile station will perform the search of thefrequencies for MAHHO when on a voice call. The search disable (SRCH₋₋DISABLE) field indicates whether a search of the MAHHO CDMA₋₋ FREQ isdisabled or that the mobile station is permitted a periodic search whenon a voice call. The (SRCH₋₋ WIN) fields are used to indicate the PNoffset and search window to be used for each neighbor pilot of theCDMA₋₋ FREQ neighbor set specified in the MNLM message channel to besearched. NGHBR₋₋ PN and SRCH₋₋ WIN are repeated as necessary.

The MNLM message is processed by DSP/MCU 228. DSP/MCU 228 storesnecessary information in memory, and configures MAHHO CTL 208 toimplement MAHHO according to the MNLM message. The MAHHO interval timeris set to the value indicated in the MNLM message appropriate for thesituation. If MS1 is involved in a voice call as in FIG. 3, and anAVER₋₋ SRCH₋₋ PRIOD has been specified along with SRCH₋₋ DISABLE set toindicate that a periodic search is allowed, the MAHHO interval timer isset to the value specified AVER₋₋ SRCH₋₋ PRIOD. If MS1 is alternativelyinvolved in a data call, and USE₋₋ TIME is enabled, the MAHHO intervaltimer would be set to the value specified in ACTION₋₋ TIME. Throughoutsteps 302 and 304 MS1 has remained on the call.

Referring now to FIG. 4, therein is shown a timing diagram illustratingreverse link transmission and handoff measurement timing according tothe embodiment of the invention. FIG. 4 shows an example of transmissionframe timing in sequences 400, 402, 404 and 406, for full rate (9600bps), half rate (4800 bps), quarter rate (2400 bps) and eighth rate(1200 bps), respectively. FIG. 4 also shows MAHHO timing in sequences408, 410, 412, 414 and 416, for the TX 210, synthesizer 206, TX 210automatic gain control, RX 204 automatic gain control and searcher 224,respectively. While MS1 is on the call each data frame output fromvariable rate vo-coder 234 is at one of full, half, one quarter or oneeighth data rates as shown in sequences 400-406.

Next, at step 306, the MAHHO interval timer expires. At step 308, inresponse to the MAHHO interval timer expiring MAHHO CTL 208 monitorstransmission frames output from vo-coder 234. When it is determined thatthe first 1/8 transmission frame has been outputted by vo-coder 234, thegate-off period of the transmission frame is checked to determine themaximum amount of time that is available for performing the MAHHOmeasurements during the transmission frame.

If a determination is made at method step 308 that the gate-off periodis greater than gate-off min, the process moves to step 310. If,however, a determination is made that the gate-off period is not greaterthan gate-off min, the process returns to step 308 and waits for thenext 1/8 rate transmission frame to be outputted from vo-coder 234. Step308 is performed once or, is repeated until the first transmission framehaving 1/8 data rate and a gate-off period greater than gate-off min isfound, and then the process moves to step 310. Alternatively, if MS1 isinvolved in a data call the step at 308, may be replaced with a step inwhich DSP/MSU 228 forces the output of VO-Coder 234 to 1/8 rate with theappropriate gate-off period. Once the data rate has been forced to 1/8rate, the process on then move to step 310. At step 310, MAHHO CTL 208waits for the last valid power control group to be transmitted in the1/8 rate transmission frame from BA1. In FIG. 4 step 310 occurs two PCGsafter PCG 2 of sequence 418, i.e., the first occurrence of datatransmission in transmission frame 406. The process now moves to step312. Step 312 occurs during PCG5 of FIG. 4. In step 312, at thebeginning of PCG5, synthesizer 206 is set to fast mode, TX 210 is gatedoff, the automatic gain control of RX 204 is tuned off, and searcher 224is either off or, alternatively, searching the current frequencies(system A frequencies) in the background. The automatic gain control ofTX210 is disabled and the current TX 210 closed loop value is stored.Next, the process moves to step 314. Step 314 occurs at the beginning ofPCG6 where the automatic gain control of RX 204 is set to the fast modeto allow the automatic gain control to track an input signal step changeof more than 40 dB.

The automatic gain control open loop estimate of RX204 is also stored.The process now moves to step 316. Step 316 occurs during PCG7 at FIG.4. In step 316, at the beginning of PCG7, the automatic gain control ofRX 204 is set to slow mode and, synthesizer 206 is set to slow mode, andsearcher 224 begins loading Buffer 222 with new frequency samples andstarts searching the PN pilots in the new frequency band.

Next the process moves to step 318. Step 318 occurs during PCG8 of FIG.4. At the beginning of PCG8, synthesizer 206 is set to fast mode, toallow RX 204 to tune to the frequency band of system A and the automaticgain control of RX 204 is set to the fast mode to allow RX 204 to trackthe input step change. The automatic gain control open loop estimate ofRX204 stored in Step 314, is restored at this point. Searcher 224continues the search of the new frequency band in the background fromdata stored in buffer 222.

Next, the process moves to step 322. Step 322 occurs during PCG9 of FIG.4. At the beginning of PCG9, the frequency error is checked to determinewhether synthesizer 206 is settled to within a predetermined range ofthe frequency f, of System A. If synthesizer 206 has settled to withinthe predetermined range, the process moves to step 326. If, however, itis determined at step 322 that synthesizer 206 has not settled to withinthe predetermined range, the process moves to step 324. At step 324 theautomatic gain control of RX 204 is set to the slow mode. The processthen waits one PCG and then moves back to step 322. At step 322 adetermination is again made as to whether synthesizer 206 has settled towithin the predetermined frequency range. The process will repeat step324 and 322 as many times as necessary for the synthesizer frequencyerror to settle.

In the example of FIG. 4, one iteration of step 324 is done in PCG 10.When the synthesizer settles, the process moves to step 326. Step 326occurs during PCG 11 in FIG. 4. At the beginning of PCG 12, TX 210 isgated on, power is restored to TX 210, synthesizer 206 is switched toslow mode, and the automatic gain control of RX 204 is set to slow modeif it is not already set to slow mode. TX 210 automatic gain control isrestored and added to the last valid TX level. MS1 may then restore openand closed loop transmit power without incurring any TX power error.

The second portion of the data transmission during the transmission fromMS1 now occurs in PCG 11. The process now moves to step 328. At step328, MS1 transmits the MAHHO pilot strength measurement message (MPSM)to base station BA1. FIG. 5B illustrates a possible configuration forthe MPSM message. The message type (MSG₋₋ TYPE) field indicates that themessage is a MPSM message. Acknowledgment sequence (ACK--SEQ) field isset by the mobile station to the (MSG₋₋ SEQ) of the most recentlyreceived formal traffic channel message requiring acknowledgment. Themessage sequence (MSG₋₋ SEQ) field is set by the mobile station to themessage sequence number for this field. The acknowledgment required(ACK₋₋ REQ) field indicates whether an acknowledgment is required forthis message. The encryption field is set to the current messageencryption mode. The CDMA frequency (CDMA₋₋ FREQ) field indicates thefrequency assignments for which the MAHHO measurements were performed.The no support (NO₋₋ SUPPORT) filed is used to indicate that the mobilestation does not support MAHHO. The reference PN (REF₋₋ PN) field is setto the PN sequence offset of the pilot used by the mobile station toderive its time reference, relative to the zero offset pilot PNsequence. The pilot strength (PILOT₋₋ STRENGTH) field is set to indicatethe strength of the pilot used by the mobile station to derive its timereference. CDMA signal strength (CDMA₋₋ SIC₋₋ STRG) is set to indicatethe CDMA signal strength on the new frequency in dBm relative to thereceived signal strength of the current CDMA frequency. KEEP indicateswhether the handoff drop timer corresponding to the pilot used by themobile to derive its time reference has expired. The pilot PN phase(PILOT₋₋ PN₋₋ PHASE) and pilot strength (PILOT₋₋ STRENGTH) fields areused to indicate the pilot measured phase and pilot strength of eachmeasured pilot. PILOT₋₋ PN₋₋ PHASE and PILOT₋₋ STRENGTH are repeated, asnecessary, for each pilot measured.

In an alternative embodiment, the MAHHO process may be done periodicallyupon the expiration of the MAHHO interval timer, without waiting for a1/8 rate transmission frame in which to make the MAHHO measurements.After the MAHHO interval timer expires similarly to Step 306, in thisembodiment TX 210 is gated off the next reverse link transmission frame,or a number of frames while MAHHO measurements are made. Forward linktransmissions are also punctured during this measurement period. Thisembodiment may be more suited to mobile stations constructed similar toMS1 at FIG. 2, that lack RX 204's automatic gain control fast mode (AGCFAST) and synthesizer 206's fast mode (SYNTH FAST) and that would not beable to perform measurements fast enough to utilize the gate-off periodof a transmission frame as described for the previous embodiment. Duringthe measurement period synthesizer 206 tunes to the search frequency. TX210 is disabled, RX 204 receives the search frequency with normalautomatic gain control, and Searcher 224 searches the new frequency onceRX 204 and synthesizer 206 have settled. When the measurements arecompleted RX 204, and synthesizer 206 return to the current frequencyand TX 210 is enabled. Searcher 224 may continue searching in thebackground. Data lost on both the forward and reverse link during themeasurement period may be recovered by normal methods.

What has been describe is a method and apparatus for Mobile AssistedHand-off between different frequencies in a Code Division MultipleAccess (CDMA) mobile cellular telephone system wherein the mobiletelephone searches for the base stations on a different carrierfrequency without significant degradation in system performance.

While the invention has been described in connection with a preferredembodiment, it is not intended to limit the scope of the invention tothe particular form set forth, but, on the contrary, it is intended tocover such alternatives, modifications, and equivalence as may beincluded within the spirit and scope of the invention as defined in theappended claims.

We claim:
 1. A cellular telecommunication system for providing MobileAssisted Hard Handoff (MAHHO) between base stations using differentfrequencies comprising:a plurality of base stations, each transmittingand receiving on forward and reverse transmission links, respectively,each base station using different forward link carrier and reverse linkcarrier frequencies; at least one mobile station for communicating witha first base station using a first forward link carrier frequency and afirst reverse link carrier frequency, and for communicating with atleast a second base station using a second forward link carrierfrequency and a second reverse link carrier frequency different fromsaid first forward carrier link frequency and first reverse carrier linkfrequency used by said first base station, said at least one mobilecellular station, each further including a MAHHO apparatus comprisingmeans for performing handoff measurements on forward link transmissionsof said at least second base station on said second forward link carrierfrequency thereof, said measurements of said forward link transmissionsof said at least second base station being performed by said MAHHOapparatus by puncturing data to be transmitted during selectedtransmission periods on said first reverse link carrier frequency tosaid first base station.
 2. A cellular telecommunication systemaccording to claim 1, wherein data transmitted to said at least onemobile station on said first forward link carrier frequency to bereceived during said selected transmission periods during said handoffmeasurements on said second forward link carrier frequency is puncturedand not received at said mobile station.
 3. A cellular telecommunicationsystem according to claim 2, wherein said at least one mobile stationMAHHO apparatus selects transmission frames including data that is to betransmitted at a selected rate for said selected transmission periodsfor said handoff measurements, said MAHHO apparatus selecting saidtransmission frames such that the position of measurement periods withinsaid selected transmission frames are chosen by said MAHHO apparatus tominimize the amount of data punctured and not transmitted by said atleast one mobile station on said first reverse link carrier frequencyduring said measurement period.
 4. A cellular telecommunication systemaccording to claim 3, wherein said MAHHO apparatus includes a timer, andwherein said selected transmission frames including data at a selectedrate are selected in response to said timer's expiration.
 5. A cellulartelecommunication system according to claim 2, wherein said at least onemobile station employs a first forward link carrier frequency of f₁ anda first reverse link carrier frequency of f₂ for communicating with saidfirst base station and a second forward link carrier frequency of f₃ anda second reverse link carrier frequency of f₄ for communicating withsaid at least second base station,said MAHHO apparatus further includingmeans for receiving signals transmitted from at least one of saidplurality of base stations; converter for converting said receivedsignals to an IF (intermediate frequency) signal, and for filtering anddigitizing said IF signal; a rake receiver connected to said converterfor correlating said converted IF signal with a desired received signal;a synthesizer having a fast and slow mode of operation; and a MAHHOcontroller connected to said synthesizer to select one of said fast andslow modes of operation.
 6. A cellular telecommunications systemaccording to claim 5, wherein:said MAHHO apparatus further includes asearcher for scanning and measuring the strength of said signalsreceived by said MAHHO apparatus, and a MAHHO pilot measurement messageprocessor connected to said searcher, said MAHHO pilot processor forprocessing said handoff measurements on forward link transmissions ofsaid at least second base station.
 7. A cellular telecommunicationssystem according to claim 6, wherein said MAHHO apparatus furtherincludes a MAHHO processor connected to said MAHHO controller to providecontrol signals to said MAHHO controller, and wherein said MAHHOcontroller controls said synthesizer and said searcher during MAHHO. 8.A cellular telecommunications system according to claim 7, wherein theoutput of said synthesizer is connected to the output of saidtransmitter.
 9. A method for performing Mobile Assisted Hard Handoff(MAHHO) between base stations having different frequencies in a cellulartelecommunications system comprising the steps of:(A) transmitting andreceiving data signals between a mobile station and a first base stationon a forward link at frequency f₁ and a reverse link at frequency f₂ ;(B) receiving, at the mobile station, MAHHO interval timer informationfrom the first base station; (C) setting a MAHHO interval timer to avalue indicated in the MAHHO interval timer information received fromthe first base station; (D) monitoring, in response to the MAHHOinterval timer expiring, transmission frames output from a vocoder inthe mobile station; (E) determining, in response to a selectedtransmission frame having a predetermined data rate being output fromsaid vocoder, the maximum time available for performing MAHHOmeasurements during said selected transmission frame, (F) activating, inresponse to determining that the maximum time period determined in StepE is greater than an off minimum value, synthesizer and searcherfunctions to perform handoff measurements on signals transmitted from asecond base station on a forward link at frequency f₃ ; and (G) sendinga MAHHO measurement message from the mobile station to the first basestation, said MAHHO measurement message including results of handoffmeasurements performed in Step E.
 10. A method according to claim 9,wherein said cellular telecommunications system comprises a CDMA system,said selected transmission frame has a predetermined data rate of a 1/8rate frame, and wherein the synthesizer and searcher functions areactivated in Step F if the maximum time of Step E is greater than agate-off minimum value.
 11. A method for providing mobile assistedhandoff in a mobile station operating in a cellular system, said methodcomprising the steps of:outputting a plurality of reverse link dataframes from an encoder, each of said plurality of reverse link dataframes sequential in time to one another, and having a transmissionperiod time during which each of said plurality of reverse link dataframes is to be transmitted to a first base station; transmitting afirst at least one reverse link data frame of said plurality of reverselink data frames to said first base station on a first frequency duringthe corresponding transmission period time of said first at least onereverse link data frame; receiving a first at least one forward linkdata frame transmitted from said first base station on a secondfrequency substantially during the corresponding transmission periodtime of said first at least one reverse link data frame; selecting asecond at least one reverse link data frame of said plurality of reverselink data frames output from said encoder; and measuring a handoffparameter on a transmission from a second base station on a thirdfrequency during the transmission period time of said second at leastone reverse link data frame selected in said step of selecting, whereinat least a portion of said second at least one reverse link data frameis punctured during the transmission period time of said second at leastone reverse link data frame, said at least a portion of said second atleast one reverse link data frame not being transmitted to said firstbase station, and at least a portion of a second at least one forwarddata frame transmitted from said first base station to said mobilestation is punctured during the transmission period time of said secondat least one reverse link data frame, said at least a portion of asecond at least one forward data frame not being received at said mobilestation.
 12. The method of claim 11, wherein said step of selectingcomprises the steps of:determining whether a data rate of a second atleast one reverse link data frame of said plurality of reverse link dataframes is less than a predetermined data rate; and, if said data rate isless than a predetermined rate; selecting said second at least onereverse link data frame of said plurality of reverse link data frames.13. The method of claim 12, wherein said step of determining comprisesthe steps of:determining whether a data rate of a second at least onereverse link data frame of said plurality of reverse link data frames isless than a predetermined data rate and whether a gate-off period ofsaid at least one reverse link data frame is greater than apredetermined gate-off period; and, if said data rate is less than apredetermined rate and said gate-off period is greater than apredetermined gate-off period; selecting said second at least onereverse link data frame of said plurality of reverse link data frames.14. The method of claim 13, wherein said method further comprises thestep of periodically generating a timer expiration signal and whereinsaid step of determining is performed in response to said timerexpiration signal.
 15. The method of claim 14 wherein said methodfurther comprises receiving a signal from said first base station, saidsignal including a timer value, and wherein said step of generating atimer signal comprise generating a timer signal according to said timervalue.
 16. The method of claim 11, wherein said method further comprisesthe step of periodically generating a timer expiration signal, andwherein said step of selecting comprises periodically selecting a secondat least one reverse link data frame of said plurality of reverse linkdata frames in response to said timer expiration signal.
 17. The methodof claim 16, wherein said method further comprises receiving a signalfrom said first abase station, said signal including a timer value, andwherein said step of generating a timer signal comprise generating atimer signal according to said timer value.
 18. An apparatus in a mobilestation for performing mobile assisted handoff between a first and asecond base station, said apparatus comprising:a coder for outputtingdata to be transmitted in a plurality of reverse link data frames, eachof said plurality of reverse link data frames sequential in time to oneanother and having an associated transmission period during which eachof said plurality of reverse link data frames is to be transmitted tothe first base station on a first frequency; a transmitter, coupled tosaid coder, said transmitter for receiving data from said coder andtransmitting said data frames of said plurality of reverse link dataframes to the first base station on said first frequency duringassociated transmission periods, said transmitter further having a firstcontrol input for receiving a first signal to disable said transmitterfrom receiving data from said coder; a receiver, said receiverconfigurable for receiving data transmitted on a second frequency in aforward link data frame from the first base station, said receiverfurther configurable for receiving a signal transmitted on a thirdfrequency from the second base station; and a MAHHO apparatuscomprising:a synthesizer, coupled to said receiver and having a secondcontrol input, said synthesizer for tuning said receiver from saidsecond to said third frequency in response to receiving a second signalat said second control input; a controller, coupled to said coder,transmitter and synthesizer, said controller for monitoring said dataoutput from said coder, detecting a selected reverse link data framethat is to be transmitted at a selected data rate, and in response todetecting said selected reverse link data frame, inputting said secondsignal to said second control input and said first signal to said firstcontrol input, causing said synthesizer to switch said receiver to saidthird frequency and disabling said transmitter from receiving data fromsaid coder during at least a portion of the transmission periodassociated with said selected reverse link data frame.
 19. The apparatusof claim 18, wherein said MAHHO apparatus further comprises:a searchercoupled to said receiver, said searcher for measuring a signaltransmitted from the second base station on said third frequency duringsaid at least a portion of the transmission period associated with saidselected reverse link data frame.
 20. The apparatus of claim 19, whereinsaid synthesizer operates in a fast or a slow mode, and wherein saidsynthesizer operates in said fast mode when switching from said secondfrequency to said third frequency and in said slow mode when saidsearcher measures said signal transmitted from the second base station.